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Inorg. Chem. 1998, 37, 2618-2619
A New Class of Luminescent Polypyridine Complexes of Rhenium(I) Containing cis-Carbonyl Ligands Erick Schutte, Jeffrey B. Helms, Stephen M. Woessner, John Bowen, and B. Patrick Sullivan* Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071-3838
ReceiVed September 19, 1997 The preparation and characterization of new luminescent metal complexes, especially those exhibiting metal-to-ligand charge transfer excited states,1 has contributed to the understanding of the fundamental principles of excited state decay. As this field has matured, applications of luminescent MLCT excited states, in particular Re(I)-polypyridine complexes containing carbonyl ligands,2 have been found in such diverse areas as solar energy conversion,3 biological labeling,4 and sensor development.5 Of specific interest to us is the discovery of new excited states which are capable of probing their immediate physical environment, for example by reporting on changes in pressure, temperature, or effective dielectric properties.6 In this Communication we report several different, high-yield routes to a new class of long-lived luminescent complexes of the type cis-Re(CO)2(N-N)(P-P)+ (where N-N is a chelate polypyridine ligand and P-P is a chelate phosphine). Also, the synthetic procedures reported here provide a new, but general, entry route to the known cis-trans-Re(CO)2(P)2(N-N)+ complexes, several of which have been prepared by other methods.7 All the new Re excited states are of extraordinary stability, exhibit spectral responses well to the red of similar tricarbonyl complexes of Re(I), and apparently do not conform to the same energy gap law as fac-Re(bpy)(CO)3L(+/0) complexes (L is a variety of neutral and anionic ligands).8 As shown in Scheme 1, three methods were used to prepare the complexes. These are route 1, reaction of fac-Re(P-P)(CO)3Cl with stoichiometric N-N and TlPF6 in o-dichlorobenzene (ODB);9 route 2, reaction of fac-Re(N-N)(CO)3Cl with stoichiometric P-P and TlPF6 in ODB;10 and route 3, reaction of fac-Re(N-N)(CO)3(1) (a) Kalyanasundaram, K. Photochemistry of polypyridine and porphyrin complexes; Academic Press: London, 1992. (b) Lees, J. Chem. ReV. 1987, 87, 711-743. (c) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, F.; Belser, P.; Von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85. (d) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193. (e) Crosby, G. A.; Highland, K. A.; Truesdell, K. A. Coord. Chem. ReV. 1985, 64, 41. (f) DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. Coord. Chem. ReV. 1985, 65, 65. (g) Watts, R. J. J. Chem. Educ. 1983, 60, 834. (2) Some leading references: (a) Wallace, L.; Rillema, D. P. Inorg. Chem. 1993, 32, 3836. (b) Worl, L. A.; Duesing, R.; Chen, P.; Della Ciana, L.; Meyer, T. J. J. Chem. Soc., Dalton Trans. 1991, 843. (c) Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.; Degraff, B. A. Inorg. Chem. 1990, 29, 4335. (d) Hino, J. K.; Della Ciana, L.; Dressick, W. J.; Sullivan, B. P. Inorg. Chem. 1992, 31, 1072. (e) Paulson, S.; Morris, K.; Sullivan, B. P. J. Chem. Soc., Chem. Commun. 1992, 1615. (f) Shaver, R. J.; Rillema, D. P. Inorg. Chem. 1992, 31, 4101. (g) Striplin, D. R.; Crosby, G. A. Chem. Phys. Lett. 1994, 221, 426. (3) Meyer, G. J., Ed. Molecular Level Artificial Photosynthetic Materials. Prog. Inorg. Chem. (Karlin, K. D., Ed.) 1996, 44. (4) (a) Oriskovich, T. A.; White, P. S.; Thorp, H. H. Inorg. Chem. 1995, 34, 1629. (b) Connick, W. B.; Di Bilio, A. J.; Hill, M. G.; Winkler, J. R.; Gray, H. B. Inorg. Chim. Acta 1995, 240, 169. (5) See, for example: (a) MacQueen, D. B.; Schanze, K. S. J. Am. Chem. Soc. 1991, 113, 6108-6110. (b) Shen, Y.; Sullivan, B. P. Inorg. Chem. 1995, 34, 6235. (c) Shen, Y.; Sullivan, B. P. J. Chem. Educ. 1997, 74, 685. (6) See, for example: (a) Vining, W. J.; Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1985, 89, 1095. (b) Lang, J. M.; Dreger, Z. A.; Drickamer, H. G. Chem. Phys. Lett. 1992, 192, 299. (c) Sullivan, B. P. J. Phys. Chem. 1989, 93, 24. (7) (a) Caspar, J. V. Ph.D. Dissertation, University of North Carolina, Chapel Hill, 1983. It was noted here that dicarbonyl Re(I) complexes had longer relative lifetimes than tricarbonyls. (b) Caspar, J. V.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1984, 23, 2104. (c) Luong, J. C. Ph.D. Dissertation, Massachsetts Institutue of Technology, 1981. (8) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952.
OTf with stoichiometric P-P in ODB.11 Route 4 in Scheme 1 is the direct preparation of complexes of the type cis-trans-Re(CO)2(P)2(N-N)+ where P can be PPh3, or even bidentate phosphines like trans-Ph(H)CdC(H)Ph that can serve as the basis for linear oligomeric chromophores.12 In all cases, reaction times for ODB heated at reflux were between 5 and 18 h, and yields after purification ranged from 60 to 80%. For all the routes, the intermediate fac-Re(bpy)(CO)3(phosphine)+ appears early in the reaction, which implies that the success of the chemistry relies on the ability of phosphine ligands to labilize a carbonyl ligand in the coordination sphere under the extreme temperature conditions. Characterization of the complexes was achieved by a combination of elemental analysis and IR and 31P NMR spectroscopies (see Table 1). As is shown in Table 1, two intense (9) All complexes of the type fac-Re(CO)3(P-P)Cl were prepared by a modification of the basic procedure found in the following: Carriedo, G. A.; Luz Rodriguez, M.; Garcia-Grande, S.; Aguirre, A. Inorg. Chim. Acta 1990, 178, 101. See also: Edwards, D. A.; Marshalsea, J. J. Organomet. Chem. 1977, 131, 73. Preparation of fac-Re(c-dppene)(CO)3Cl: [Re(CO)5Cl] (1.0 g) and 1.15 g of c-dppene were combined in a 100 mL round bottom flask containing ca. 50 mL of deoxygenated toluene. The solution was then placed under an N2 blanket and refluxed for 6 h. After this time the white precipitate was filtered from the hot mixture. The precipitate was washed with another 50 mL of hot toluene and discarded. The filtrate was reduced to ca. 20 mL by rotary evaporation and poured slowly into 100 mL of stirred ether. The white precipitate that formed was washed with 50 mL (3×) of ether and then dried. Preparation of cis-[Re(CO)2(c-dppene)(phen)]PF6: fac-Re(cdppene)(CO)3Cl (100 mg), 53 mg of TlPF6 (7% excess), and 28 mg of 1,10-phenanthroline (10% excess) were combined in a 50 mL round bottom flask which was covered with aluminum foil. Approximately 5 mL of o-dichlorobenzene was added, and solution was refluxed for 7 h, resulting in a yellow-orange color. The solution was allowed to cool, followed by filtration over diatomaceous earth to remove the chalkywhite TlCl precipitate. This was washed with ca. 100 mL of CH2Cl2, and the filtrate was allowed to evaporate over a 48 h period. Ethyl ether was added to cause precipitation of the yellow product, which was subsequently filtered off and washed with 50 mL of ethyl ether. Anal. Calcd: C, 49.74; N, 2.90; H, 3.34. Found: C, 49.93; N, 2.83; H, 3.15. (10) Preparation of [cis-Re(CO)2(dppm)(bpy)]PF6 from fac-Re(bpy)(CO)3Cl: fac-Re(bpy)(CO)3Cl (100 mg), 76 mg of TlPF6, and 91 mg (10% excess) of dppm were combined in a 50 mL round bottom flask which was covered with aluminum foil. Approximately 7.5 mL of o-dichlorobenzene was added, and N2 was bubbled through the solution for 5 min, which was then placed under a N2 blanket and refluxed for 5 h. During this time the solution changed from white to orange-red. The reaction workup was exactly the same as that for cis-[Re(CO)2(c-dppene)(phen)]PF6. Anal. Calcd: C, 47.90; N, 3.02; H, 3.26 Found: C, 48.02; N, 3.09; H, 3.31. (11) Preparation of [cis-Re(CO)2(dppm)(bpy)](OTf) from fac-Re(bpy)(CO)3OTf: Re(bpy)(CO)3(OTf) (155 mg, 0.27 mmol) and dppm (104 mg, 0.27 mmol) were placed in a 50 mL round bottom flask containing 10 mL of o-dichlorobenzene. The reaction mixture was purged with nitrogen for 30 min and refluxed for 5 h, after which time 40 mL of diethyl ether was added, following cooling. The yellow-orange solid which precipitated was collected and washed with ether, resulting in an orange oil. This oil was dissolved in dichloromethane and dropped into stirring ether, giving the yellow-orange product, which was collected, washed with ether, and air-dried (188 mg, 73% yield). (12) Woessner, S.; Sullivan, B. P. Work in progress. (13) (a) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys. Chem. 1986, 90, 3722. (b) Henry, B, R.; Siebrand, W. In Organic Molecular Photophysics; Birks, J. B., Ed.; Wiley: London, 1973; Vol. 1, Chapter 4. (c) Avouris, P.; Gelbart, W. M.; El-Sayed, M. A. Chem. ReV. 1977, 77, 793. (d) Freed, K. F. Acc. Chem. Res. 1978, 11, 74. (e) Lin, S. H. Radiationless Transitions; Academic Press: New York, 1980. (f) Heller, E. J.; Brown, R. C. J. Chem. Phys. 1983, 79, 3336.
S0020-1669(97)01199-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/07/1998
Communications
Inorganic Chemistry, Vol. 37, No. 11, 1998 2619
Scheme 1. New Preparative Reactions (Routes 1-4; See Text) for the cis-Carbonyl Complexes
Table 1. Spectral and Photophysical Data for the Complexesa,b no.
complex
1 2 3 4 5 6
cis-Re(CO)2(dppm)(bpy)+ cis-Re(CO)2(c-dppene)(bpy)+ cis-Re(CO)2(c-dppene)(phen)+ cis-trans-Re(CO)2(t-dppene)2(bpy)+ cis-trans-Re(CO)2(dppm)2(bpy)+ cis-trans-Re(CO)2(PPh3)2(bpy)+
31P
NMR shifts (ppm)c -30.7, -18.7 +40.0, +49.9 +41.1, +51.3 +16.6, -5.7 -28.6, +14.5 +24.6
IR freq (cm-1)
Eop (nm)d
Eem (nm)d
τ (ns)e
φf
1950, 1884 1956, 1886 1960, 1890 1942, 1873 1940, 1869 1938, 1868
439 (3.53) 389 (3.56) 390 (3.70) 420 (3.38) 428 (3.45) 425 (3.54)
642 612 603 637 634 620
768 378 3840 191 199 775g
0.020 0.033 0.123 0.014 0.014 0.039g
a All complexes are triflate salts in CH Cl except complex 3. b Abbreviations: dppm is bis(diphenylphosphino)methane, c-dppene is 2 2 cis-(bis(diphenylphosphino))ethylene, t-dppene is trans-(bis(diphenylphosphino))ethylene, bpy is 2,2′-bipyridine, and phen is 1,10-phenanthroline. c In CH CN with an 85% phosphoric acid external standard. d E and E 3 op em are respectively the absorption and emission spectral maxima corresponding to the lowest energy MLCT excited state (log max is reported). e Measured by the phase shift demodulation method with a glycogen scattering standard. Errors are (5%. f Measured against an air-saturated sample of fac-[Re(bpy)(CO)3py]OTf in water (φ ) 0.012). g Reported value of 801 ns as a PF6- salt in CH2Cl2 (Caspar, J. V. Ph.D. Dissertation, 1982).
CO vibrational modes in the region of ca. 1870-1890 and 19401960 cm-1 and the appropriate number of 31P NMR spectral peaks are consistent with the structural assignments of the complexes. All complexes exhibit a yellow to red-orange coloration in the solid and in solution that is due to MLCT transitions in the 390440 nm region. The corresponding luminescence occurs in the ca. 600-640 nm range (see Table 1). Of special interest from a photochemical perspective are the complexes cis-[Re(CO)2(dppm)(bpy)]OTf (1) and cis-[Re(CO)2(c-dppene)(phen)]PF6 (3) (dppm is bis(diphenylphosphino)methane and c-dppene is cis1,2-(bis(diphenylphosphino))ethylene). Complex 1 possesses spectral and photophysical parameters similar to those of Ru(bpy)32+; for example, compare the energy maxima for absorption and emission and the lifetimes and quantum yields of the lowest MLCT excited state for 1 (440 nm, 642 nm, 768 ns, 0.029; CH2Cl2) and Ru(bpy)32+ (449 nm, 607 nm, 490 ns, 0.024; CH2Cl2).7a Complex 3 exhibits a lifetime of 3.84 µs and a quantum efficiency of emission of 12.3% (CH2Cl2), both of which are extraordinary in the photochemistry of MLCT excited state chromophores. The new complexes exhibit surprisingly long lifetimes relative to the fac-Re(bpy)(CO)3Ln+ series (L is Cl- (n ) 0) or L is a neutral nitrogen or phosphorus donor (n ) 1)). For example, complex 1 and cis-[Re(CO)2(c-dppene)(bpy)]OTf (2) exhibit lifetimes of 768 and 378 ns, respectively. This difference is primarily due to a smaller than expected nonradiative rate constant, knr.13 In the energy gap law analysis published by Caspar and Meyer for fac-Re(bpy)(CO)3Ln+ the linear relationship ln knr ) 40.155 - 1.458Eem was found.8 On the basis of this finding,
the calculated knr values for 1 and 2 are 1.13 × 107 and 3.80 × 107 s-1 versus the observed values of 1.27 × 106 and 2.56 × 106 s-1. Given the relatively constant value of kr (ca. 105 s-1) characteristic of the fac-Re(bpy)(CO)3Ln+ series, the calculated knr values predict lifetimes of 26 and 85 ns for 1 and 2. Several explanations are possible for this behavior. One is the presence of higher-lying states that can be thermally populated, leading to either longer lifetimes for the dicarbonyls or shorter ones for the tricarbonyls depending on the identity of the state (e.g., ligandlocalized versus d-d).14 Another is an inherent difference in Franck-Condon factors. Since some studies have shown that medium-frequency bpy modes, in addition to higher CO modes, act as energy acceptors,2b one possible cause of the dramatically increased relative lifetimes is the removal of a critical high-energy mode that participates in energy disposal and its replacement with fewer modes of smaller quantum spacing. Our future work will be aimed at resolving this issue. An important point to remember, however, is that regardless of origin, the room temperature energy gap law behavior is an empirical yardstick by which MLCT excited states may be judged for their utility in numerous applications. Acknowledgment. B.P.S. thanks the DOD (AFOSR) and NSF EPSCoR programs and Dr. John Endicott for helpful discussions. IC971199S (14) A meaningful comparison of knr values can be made only if the potential temperature dependence of this rate constant is taken into account. Studies of the temperature variation of knr for both the tricabonyls and dicarbonyls must be done before any conclusion concerning the origin of the lifetime differences can be drawn.
